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Kevin,

We have to realize that oxygen is carried in 2 ways, both as bound
oxygen with hemoglobin, and as dissolved oxygen.  Hemoglobin carries
a tremendous amount of oxygen per mmHg oxygen partial pressure (somewhere
around 20 ml of oxygen for each 100ml of blood with a hemoglobin of 
15 grams/100 ml).  Thus, at sealevel, we see an oxygen consumption of
around 5 volumes percent (5ml of oxygen extracted from every 100ml of
blood), and this drops the oxygen partial pressure from around 100mmHg
on the arterial side to around 60-70mmHg on the venous side.  Thus, 
breathing air at sealevel, we have an oxygen window of 30-40mmHg.

Now, as we increase the partial pressure of oxygen in the inspired gases,
hemoglobin become essentially 100% saturated with oxygen at around 150mmHg
oxygen partial pressure.  With oxygen partial pressures higher than that, the
only effect is to increase the amount of dissolved oxygen carried in the 
liquid part of the blood (ie, plasma).  Oxygen is fairly poorly soluble
in liquids, so it is dissolved at the rate of 0.003ml of oxygen per mmHg
per 100ml of blood.    

Thus, we start out at sealevel on air with an oxygen content of 20.3 ml
oxygen per 100 ml of blood, with a arterial oxygen partial pressure of
approximately 100mmHg.  At 150mmHg, the hemoglobin is 100% saturated, and
beyond we only increase the dissolved oxygen.  When the tissues use this
oxygen, it consumes the dissolved oxygen until the blood oxygen partial
pressure gets down to around 100-150mmHg, at which point the oxygen combined
with the hemoglobin begins to be released.

Thus, if we are breathing 100% oxygen at 2ATA, we are looking at an arterial
partial pressure of oxygen of around 1500 mmHg (factoring in CO2 partial
pressures, etc.).   If we assume that the oxygen consumption remains at 
5ml per 100ml blood, then the arterial oxygen partial pressure will drop
from around 1500 to around 90mmHg in the venous blood.  This produces an
oxygen window of roughly 1400mmHg.  Once we get past the point where
the entire oxygen requirements are supplied by the dissolved oxygen, the
oxygen window becomes fixed (if cardiac output continues to compensate
for metabolic rate).  

So, for values of oxygen partial pressure that are less than around 1600mmHg,
the oxygen window will vary with the inspired oxygen partial pressure.  The
higher (up to about 2.1-2.2 ATA) the inspired oxygen partial pressure, the
larger the oxygen window.

John

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